Impedance matched open-ended waveguide array

ABSTRACT

The apparatus of the present invention provides an open-ended waveguide array wherein the impedance of the array elements are efficiently matched over a wide beam scan angle at reasonable cost. This impedance matching is achieved by employing open-ended waveguide radiators which are capable of supporting multiple waveguide modes for the respective radiating elements. In the operation of an illustrative embodiment of the array, the excitation of the two lowest order modes of appropriate phase in each of the open-ended waveguide radiating elements produces well-matched elements over a wide beam scan angle.

O United States Patent [1113,573,s3s

[72] Inventors Louis Stark [56] References Cited Fullerton; UNITED STATES PATENTS f a Anabel; Nam 2,349,942 5/1944 Dallenbach 343/776 [211 App! N0 5 3,495,062 2/1970 Puschner 343/778 a [22] Filed J 1969 3,496,571 2/1970 Walter et a1. 343/771 [45] Patented Apr. 6, 1971 Primary Examiner-E11 Lieberman [73] Assignee Hughes Aircraft Company Attorneys-James K. Haskell and Robert H. Himes Culver City, Calif. M

ABSTRACT: The apparatus of the present invention provides an open-ended waveguide array wherein the impedance of the [54] IMPEDANCE MATCHED OPENENDED array elements are efficiently matched over a wide beam scan angle at reasonable cost. ThlS impedance matching is achieved WAVEGUIDE ARRAY 2 Claims 4 Drawin Fi s by employing open-ended waveguide radiators WhlCh are g g capable of supporting multiple waveguide modes for the [52] US. Cl 343/776, respective radiating elements. In the operation of an illustra- 343/786, 343/846 tive embodiment of the array, the excitation of the two lowest [51] Int. Cl. H0lq 13/00 order modes of appropriate phase in each of the open-ended [50] Field of Search 343/772, waveguide radiating elements produces well-matched elements over a wide beam scan angle.

IMPEDANCE MATCHED OPEN-ENDED, WAVEGUIDE ARRAY BACKGROUND OF THE INVENTION Contemporary techniques for improving the impedance match over a range of scan angles include, for example, the placement of dielectric sheets in front of the array aperture, the interconnecting of array elements with coupling circuits, or the use of parasitic elements or chokes. Antenna systems employing these techniques are comparatively less efficient IIOII.

SUMMARY OF THE INVENTION In accordance with the present invention, a linear array or a two-dimensional array of open-ended waveguide radiators, each capable of supporting two waveguide modes, are employed. The two waveguide modes are excited in the respective radiating elements by utilizing a coaxial to waveguide feed with offset capacitive or inductive posts. Each of these modes excites an evanescent surface wave along the array aperture when all terminals are open circuited and the superposition of the two modes yields the correct shape of mutual impedances over the array which produces a constant driving-point impedance over a specified scan angle when all elements in the array are driven.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a partially cutaway perspective view of a linear array having impedance matched waveguide-radiating elements with offset loop transitions;

FIG. 2 shows a top view of a single waveguide-radiating element of the linear array of FIG. 1 illustrating the offset of the end-on loop transition;

FIG. 3 shows a perspective view of an alternate waveguideradiating element for the linear array of FIG. 1 illustrating an offset probe transition; and

FIG. 4 shows a top view of the alternate waveguide-radiating element of FIG. 3.

DESCRIPTION Referring now to FIG. I of the drawing, there is shown a linear array in accordance with the present invention mounted on a groirnd plane 12. The radiating elements of the linear array 10 constitute a plurality of open-ended rectangular waveguide elements 14-19 arranged to expose a portion of the length thereof above the ground plane 12 with the broad walls thereof spaced at uniform intervals of less than one-half free space wavelength and corresponding narrow walls thereof disposed in common planes. Each of the openended rectangular waveguide elements 14-19 have shorting plates disposed transversely across the extremity thereof opposite from the open end and are fed therethrough by coaxial lines 21-26, respectively. The center conductors 30 of the coaxial lines 21-26 extend into the waveguide elements 14- -19, respectively, to provide offset-loop transitions in the manner hereinafter described. The overall length of the waveguide elements 14-19 is not critical and is, for example, of the order of three guide wavelengths.

Referring to FIG. 2, there is shown a top view of the openended rectangular waveguide element 14. The shorting plate 20 has a circular opeiiing 28 adapted to accommodate the outer conductor of input coaxial line 21 and is centered of the order of 0.l times the width w of the waveguide element 14 from a center line normal to the broad walls thereof. The center conductor 30 of the coaxial line 21 extends one quarter guide wavelength into the waveguide element 14 prior to making a loop transition, i.e., making a right angle turn and connecting perpendicularly to a broad wall thereof. Because of symmetry, the coaxial lines 21-26 may connect to the waveguide elements 14-19, respectively, on either side of the centerline and may connect to either broad wall. The orientaand more costly to fabricate'than' the apparatus of the invention of the coaxial lines 21-26 should, however, be consistent in each of the waveguide elements 14-19. In addition, although an offset of 0.1 of the width of the waveguide has been specified, the optimum offset is a function of the spacing between the waveguide elements in the linear array. For an element spacing of half the free space wavelength, an offset of 0.1 the width will substantially reduce variations in the driving point impedance of the linear array, thus providing a matched aperture over a wide scan angle.

In operation, each coaxial line 21-26 of the linear array 10 is driven by a phase shifter and a multiple output feed system in the conventional way. The offset loop of the center conductors 30 of the coaxial lines 21-26 excite the two lowest order modes in the open-ended waveguide elements 14-19. Each of these modes excites an evanescent surface wave along the linear array 10 when all terminals are open circuited which two modes superimpose to yield the correct shape of mutual impedances between the driven element and its neighboring elements. These mutual impedances produce a substantially constant driving-point impedance at each element coaxial input terminal over a specified scan angle when all of the elements 14-19 of the linear array 10 are driven. From the standpoint of a scattering formulation instead of an impedance formulation, the modes produced in the multimode elements 14-19 are reflected from the respective apertures thereof and are recoupled to the driving terminal to produce a net reflected wave in the transmission line connecting to the terminal. Each mode in the total array possesses a characteristic reflection coefiicient vs. scan angle at the aperture and produces a corresponding contribution to the reflected wave in the terminal line. Wide-angle impedance matching of the respective radiating elements 14-19 is achieved by a cancellation of the aforementioned aperture reflections of the two modes at the junction (not shown) of the coaxial lines 21-26.

Referring now to FIGS. 3 and 4 of the drawing, there is shown an open-ended waveguide-radiating element 32 with an offset probe 34 for exciting the TE and the TE modes therein, i.e., the two lower order modes. The probe 34 extends across the narrow dimension of the waveguide, is ofiset approximately 0.] the width from the center thereof and is located one quarter guide wavelength from a shorting plate 35 disposed transversely across the extremity of the waveguide element 32 opposite from the open end. As before, the overall length of the waveguide element 32 is of the order of three guide wavelengths. A circular opening is disposed in a broad wall of the waveguide symmetrically about the probe 34 to enable the outer conductor of coaxial line 36 to terminate at the broad wall and the center conductor thereof to connect to the probe 34.

We claim:

1. An antenna array comprising a plurality of open-ended rectangular waveguide elements of uniform length having narrow walls and broad walls, said waveguide elements being disposed at uniform intervals in a linear array with said broad walls thereof normal to the direction thereof and with one corresponding extremity of each of said open-ended rectangular waveguide elements disposed in a common plane; a shorting plane disposed transversely across each of said plurality of waveguide elements at the respective extremities thereof opposite from said one extremity; a coaxial line input to each of said rectangular waveguide elements; and an offset-loop transition between each respective waveguide element and the coaxial line corresponding thereto, the offset from center along said broad walls of said loop transition between respective waveguide elements and corresponding coaxial lines center along said broad walls of said probe transition between respective waveguide elements and corresponding coaxial lines being greater than 5 percent of the width of said broad walls. 

1. An antenna array comprising a plurality of open-ended rectangular waveguide elements of uniform length having narrow walls and broad walls, said waveguide elements being disposed at uniform intervals in a lineAr array with said broad walls thereof normal to the direction thereof and with one corresponding extremity of each of said open-ended rectangular waveguide elements disposed in a common plane; a shorting plane disposed transversely across each of said plurality of waveguide elements at the respective extremities thereof opposite from said one extremity; a coaxial line input to each of said rectangular waveguide elements; and an offset-loop transition between each respective waveguide element and the coaxial line corresponding thereto, the offset from center along said broad walls of said loop transition between respective waveguide elements and corresponding coaxial lines being greater than 5 percent of the width of said broad walls.
 2. An antenna array comprising a plurality of open-ended rectangular waveguide elements of uniform length having narrow walls and broad walls, said waveguide elements being disposed at uniform intervals in a linear array with said broad walls thereof normal to the direction thereof and with one corresponding extremity of each of said open-ended rectangular waveguide elements being disposed in a common plane; a shorting plane disposed transversely across each of said plurality of waveguide elements at the respective extremities thereof opposite from said one extremity; a coaxial line input to each of said rectangular waveguide elements; and an offset probe transition between each respective waveguide element and the coaxial line corresponding thereto, the offset from center along said broad walls of said probe transition between respective waveguide elements and corresponding coaxial lines being greater than 5 percent of the width of said broad walls. 